Ignition

Sources of Ignition #

Sources of ignition are many and varied and include:

The sources of ignition as far as this course is concerned are primarily from electrical equipment.

Some of the above sources of ignition are examined in detail below.

Electrical Sparks #

Electrical sparks are caused primarily by the opening and closing of contacts in electrical switches, contactors and relays. To ignite a flammable mixture consisting of hydrogen and air requires only 20µJ, the energy produced as a result of a break of 0.1mS duration in a circuit carrying 20mA at 10V.

Flammable gases and vapours are more readily ignited at high voltages than low voltages, and this is basically why IS circuits are seldom designed for use above 30V.

For sparking devices to operate safely in a hazardous area, they must installed in a flameproof enclosure, for example.

The use of electrical test instruments e.g. meters and insulation resistance testers, are a potential source of sparks. Such instruments should only be used under a permit to work system and after testing to ensure gas free conditions.

Hot Surfaces #

The flow of current through the windings of motors or other electrical equipment invariably produces heat which will raise the surface temperature. If the motor is excessively overloaded and the thermal overload device in the starter is incorrectly set, the surface temperature of the motor may well exceed its “T” class (see later sections).

Overheating can also be caused by blockage of the cooling fan intake, damaged colling fan, or collapse of bearing due to lack of lubrication. This can dramatically raise the surface temperature locally to a ‘blue heat’ state with a temperature of around 430°C which is capable of igniting most flammable gas or vapour.

Other sources of heat are process pipes and machinery, combustion engine manifolds and exhaust pipes, and light bulbs.

Batteries #

Batteries, whatever their size, are a potential source of ignition as they will produce incendive sparks if their terminals are short-circuited. Current of the order 1000A can be generated if the terminals of automotive batteries ae short-circuited.

There is the added complication that, during charging of lead-acid batteries, hydrogen and oxygen are released. This requires well ventilated battery rooms.

The certification of portable instruments may only allow their use in hazardous areas if powered by low-power batteries. These must not be replaced with other types unless permitted by the manufacturer. Replacement of batteries must only be carried out in a non-hazardous area.

Friction #

The abrasive wheels of portable grinding machines can of produce incendive sparks and hot surfaces locally at the point of contact by the abrasive wheel. Drilling using portable tools can also generate heat between the drill bit and the work piece.

Power tools must not be used in hazardous areas, unless used under strictly controlled conditions, because they themselves are sources of ignition from sparking commutators etc.

Static Electricity #

Static electricity is normally caused by two materials rubbing together. The loosely held electrons in the atoms of one material are detached and transferred to the other materials, so that the material which loses electrons becomes [positively charged. This condition may remain for some time because the materials are insulators and do not offer a conductive return path for the electrons.

Nylon (and other synthetic fibre) clothing removed from the body can generate enough static electricity to ignite a flammable gas or vapour.

Plastic explosion protected enclosures normally carry the waring that they should be cleaned using a damp cloth to avoid generation of static electricity. Such cleaning may be covered by ‘special conditions of use’ in the equipment certificate.

The movement of fluids can also generate electrostatic charges, and kilovolts can be generate at the nozzle of aerosols or high-pressure cleaning equipment.

Bonding and earthing of aircraft and hose nozzle during refuelling prevents the build-up of electrostatic charges which might otherwise cause the aviation fuel vapour to ignite.

Lightning #

Lightning is a type of static electricity caused by the movement of clouds. Air between clouds or between clouds and earth acts as an insulator allowing the charges to build up, and the result is that very high voltages are generated.

Once the voltage reaches a critical point, breakdown of the air occurs and the energy is released suddenly in the form of a lighting strike. Lighting strike will be readily discharged to earth by the normal metal construction of an installation, but flammable gases or vapours can be ignited by lightning.

Thermite Reaction #

The impact between rusty iron or steel and aluminium or other light metals can produce sparks capable of igniting a flammable gas or vapour. In this process, the sparks are produced by a reaction know as a thermite reaction.

The use of aluminium ladders in hazardous areas should therefore be avoided. The use of aluminium paint in hazardous areas also requires caution.

Pyrophoric Reaction #

Hydrogen sulphide (H2S) or other sulphide compounds passing through iron pipes react with the iron of the pipe to produce iron sulphide. Iron sulphide when exposed to air very quickly oxidises and will reach temperatures capable of igniting a flammable gas or vapour.

This phenomenon is known as pyrophoric reaction and be prevented by soaking the iron sulphide with water or preventing its contact with air.

Radio frequency (R.F) #

The use of mobile telephones at petrol filling stations is considered a risk.

Petrol stations have Zone 1 areas around the pumps, and the energy transmitted by a mobile phone could be picked up by metalwork in the area which, acting as an aerial, could produce a spark of sufficient energy to ignite the vapour.

Radar, radio and television transmitters are powerful sources of R.F.

Optical radiation #

High energy light sources such as lasers are another possible source of ignition, and standard IEC60078-28 “Protection of optical radiation systems” was issued in 2006. A break in a fibre-optic cable could be a potential ignition source.

Vibration #

Vibration is undesirable since it causes premature deterioration of equipment if allowed to persist. Typical examples are increased wear in bearings, loosening of electrical connections, etc. Local overheating can quickly occur in these conditions.

Vibration has also been known to cause metal fatigue of the copper sheath and conductors of MICC cable due to work hardening.